1. Field of the Invention
The present invention relates generally to a snub valve and, more particularly, to a snub valve for a hydraulic system which provides a progressively reduced flow of fluid through the valve in response to the movement of a movable object toward a predetermined end-of-travel position.
2. Description of the prior art
In systems which use hydraulic power to move a movable object, such as a flight control surface of an airplane, a door or other device which is moved in response to the flow of a motive fluid, it is advantageous to provide a means for slowing the rate of movement of the movable object as it approaches its end-of-travel position. For example, if a flight control surface is moved by hydraulic power from a fully opened position to a fully closed position, the flight control surface may possibly be damaged if it is permitted to move at full speed against a mechanical stop at its end-of-travel position.
Various types of flow control valves are known to those skilled in the art. For example, U.S. Pat. No. 3,319,653, which issued to Stephens on May 16, 1967, discloses a flow control valve with a spool valve portion which incorporates an integrally housed anticavitation valve and a load check valve within the body of the flow control valve. This valve provides a means by which the return oil from the working cylinder is fed into the pump flow when negative pump pressure is indicated by the loading of a work cylinder.
U.S. Pat. No. 3,216,446, which issued to Schmiel on Nov. 9, 1965, discloses a spool valve assembly with dual check valves. The check valves in this device are arranged to prevent back flow of fluid from a motor port to the pressure inlet port of the spool valve housing and to permit flow of fluid from a tank port to a motor port. The purpose of this arrangement is to prevent load dropping and cavitation of a fluid motor that is controlled by the spool valve assembly.
U.S. Pat. No. 3,866,627, which issued to Dezelan on Feb. 18, 1975, discloses a dual check valve arrangement in which a first check valve is positioned between a pump and a selector valve to pass pressurized hydraulic fluid thereby and a second check valve that is mounted telescopically within the first check valve. The second check valve is opened when the fluid pressure in the reservoir exceeds a predetermined magnitude.
U.S. Pat. No. 3,661,422, which issued to Sember et al on May 9, 1972, discloses an adjustment valve for use in controlling air flow into open-cell foam when the foam is under a vacuum condition.
Copending patent application Ser. No. 07/046,471, filed on May 4, 1987 by Koerber, discloses a hydraulic snub valve that incorporates a movable piston with a bypass conduit formed through the piston for the purpose of providing a bypass from the inlet of the valve to the outlet of the valve when the piston has otherwise been moved into a blocking position to stop the main flow of fluid through the valve. The function of this hydraulic snub valve is similar to the function of the present invention. It is intented to permit a full rate of fluid flow through the valve until a predetermined period of time has elapsed. After the predetermined period of time has elapsed, flow through the valve is restricted to that which can pass through the bypass conduit. This has the effect of permitting a movable object to be moved at its maximum rate of speed until the predetermined time period has elapsed and, after that occurrence, to permit a lessor flow of fluid through the valve for the purpose of moving the movable object toward its end-of-travel position at a slower speed.